1. Field of the Invention
The invention relates to a device for supplying a liquid additive. Devices of this type are used, for example, in the automotive sector to feed a liquid additive to the exhaust-gas treatment device of an internal combustion engine.
2. Related Art
For example, widespread use is made of exhaust-gas treatment devices in which nitrogen oxide compounds in the exhaust gas of an internal combustion engine are reduced with the aid of a reducing agent. The corresponding exhaust-gas purification method is referred to as the SCR (Selective Catalytic Reduction) method. Ammonia is typically used as reduing agent. The ammonia reacts with the nitrogen oxide compounds on an SCR catalytic converter in the exhaust-gas treatment device.
Ammonia is normally fed to the exhaust-gas treatment device not directly but rather in the form of a precursor solution that can be stored and supplied as liquid additive. The precursor solution is converted, in the exhaust gas, into ammonia. A particularly widely used precursor solution is urea-water solution, which is available for example under the trade name AdBlue® with a urea content of 32.5%.
For the delivery of liquid additive, a device for supplying liquid additive typically has a pump. A pump of this type should, on the one hand, be as inexpensive as possible and, on the other hand, permit highly reliable operation of the device. The operation of the device gives rise to various demands on the pump. Firstly, it should be possible for the delivery rate of the pump to be adapted during operation to different operating conditions of the exhaust-gas treatment device.
Furthermore, it may be necessary for the delivery pressure generated by the pump to correspond as precisely as possible to a predefined pressure range.
At the same time, a device for supplying liquid additive must, even in the presence of frozen additive, be ready for use again quickly. A urea-water solution freezes, for example, at temperatures below −11° C. When this happens, a volume expansion occurs, which can damage the lines in a device for supplying liquid additive. In motor vehicles, such low temperatures arise in particular during long standstill phases. A device for supplying liquid additive should thus be designed and/or set up such that it is not damaged by the volume expansion of the liquid additive during the freezing process. This may be realized, for example, by virtue of the device being evacuated upon a stoppage of operation. It is also possible for the device to be designed so as to compensate the volume expansion.